The proposed research seeks to identify the luteotropic hormone(s) and factors which initiate and/or inhibit luteal function and nidation in western spotted skunks. In addition, I propose to determine whether or not there is a change in luteal cell affinity for gonadotropins throughout pregnancy. Pregnant skunks will be pretreated during the prolonged preimplantation period with varying doses of estradiol and FSH in hopes of enhancing luteal response to increased endogenous gonadotropin secretion (induced by LH-RH). Experiments to determine which hormones are luteotropic will be conducted by constantly infusing these hormones individually and in combination for 120 min and measuring plasma progesterone levels at 0, 20, 40, 60, 90 and 120 min. To confirm the luteotropic role of the hormones tested in previous experiments, 125I or 3H labelled hormones will be injected into pregnant skunks and the corpora lutea tested for selective uptake of radioactivity. If selective luteal binding of gonadotropic hormones is achieved, an attempt will be made to further test the hypothesis that luteal involution during obligate delay of implantation results from insufficient luteal affinity for gonadotropic hormones. Hysterectomies will be performed during the pre and postimplantation periods to test the hypothesis that the uterus and/or embryo secrete a substance which inhibits luteal function during delay of implantation or enhances luteal function after nidation.